The present invention relates to the art of microporous materials, especially zeolites, and, in particular, to new methods of treating such materials, and to the products resulting from such treatment.
Certain microporous materials, especially zeolites, are unique minerals which were formed millions of years ago as a result of volcanic activity beneath ancient desert lakebeds. The awesome forces of nature combined to form this remarkable family of minerals which absorb and release water vapor and absorb specific gas molecules. The physical structures of zeolites and similar microporous materials are arranged in an interconnecting framework structure. This structure is arranged to form a honeycomb framework of interconnecting channels that are consistent in diameter. The diameter of these open channels is what differentiates each type of microporous material, such as the zeolite family, and is what gives rise to their unique properties. Within these channels are positively charged ions (cations) attached and held by the framework's negative charge.
Microporous materials, especially zeolites, can be used to perform a variety of functions. They can be used for water absorption/desorption and have the ability to absorb/desorb water vapor without toplogical change in the interconnecting framework structure. Some microporous materials, such as zeolites, also have the ability to selectively absorb specific gas molecules without any effect. In addition, microporous materials, most notably zeolites, are some of the most efficient ion exchangers known. They have the ability to exchange one cation for another determined by ion size and channel diameter.
Zeolites are naturally occurring aluminosilicate materials crystallizing in a variety of low-density framework structures constructed from corner-connected (Al,SiO4)-tetrahedra. These units define windows with a narrow size-distribution of pores and channels of molecular dimensions. It is the restricted access to the interior that provides the reactant-, transition state and product-selectivity. This selectivity makes these “nanoreactors” valuable selective heterogeneous catalysts and ion exchangers in a number of industrial and environmental applications. The built-in flexibility of the T-O-T angle connector between tetrahedral units allows these structures to contract and expand in response to thermodynamic variables such as temperature and pressure. Other microporous materials have structures similar to zeolites and share many of the same properties of zeolites. These microporous materials are often referred to as zeolite-like materials.
While an ever-expanding variety of microporous materials with a wide range of pore sizes is available, it is desirable to have a way to vary the chemistry of the nanopores for a given framework topology and provide selective access to the interior for ion exchange and sorption. Temperature has been used almost exclusively to control the degree of hydration and hydroxylation, remove templating molecules after synthesis, facilitate ion exchange or gas separation processes or to control the cation distributions within the pores. However, modifications of the nanopores using temperature may compromise the mechanical integrity of the whole atomic scaffolding, and indeed, in many cases the metastable frameworks collapse to dense structures below the corresponding glass transition temperatures, the upper limit for the temperature-driven applications. For those classes of microporous materials with relatively dense framework structures, such as natrolite and related analogues, the limited access to the internal pores makes facile and reversible tuning of the nanopore chemistry, using temperature alone, difficult.
Accordingly, there is a need for a way to vary the chemistry of the nanopores for a given framework topology without damaging or destroying the interconnecting framework structure.